A nuclear reactor core is typically constructed as an array of fuel assemblies (FA's) in which each FA is vertically coextensive with the height of the reactor core and the array of FA's spans the lateral dimensions of the reactor core. Each FA comprises an array of vertically oriented fuel rods held together by a structural skeleton comprising a set of horizontal spacer grids spaced apart along the vertical direction which are welded to guide tubes or other rod-like vertical elements. The upper and lower ends of the FA are capped by upper and lower end fittings (also sometimes called nozzles) connected to the guide tubes by fasteners, welding, or the like.
Typically, the spacer grids are made up of interlocking straps, where each strap is machined (e.g. stamped) from a strip of metal, such as a nickel-chromium alloy (e.g., Inconel™) strip or a zirconium alloy (e.g., Zircaloy™) strip. The intersecting straps define openings, also called cells, through which fuel rods pass. The straps are machined or stamped to define dimples (i.e., “hard” stops, protrusions having high stiffness), springs (i.e. “soft” stops, protrusions having low stiffness), or other retention features to hold the fuel rods passing through the spacer grid.
Prior to welding, the assembly of interlocked straps is relatively loose, and is not structurally rigid. Therefore, the assembly of interlocked straps is mounted in a spacer grid welding fixture during the welding process.
Existing spacer grid welding fixtures typically follow one of two methods for holding the straps in position for the welding. The first approach uses pins that are inserted into the cells of the spacer grid. The pins have suitably machined surfaces that hold the straps in position. The pins are also machined to clear stamped features and the intersection weld. The second approach employs fixture plates. The spacer grid is inserted into the fixture plate which positions the straps by contacting strap features. Typically the solid and slotted sides of the straps are both held by the fixture.
These existing grid welding fixtures have substantial disadvantages. They are difficult to machine. Inserting the assembled (but not yet welded) spacer grid into the fixture is difficult, because the entire interlocking assembly is inserted as a unit. Prior to welding, the slotted straps are flexible and may be damaged if not properly aligned in the welding fixture. Strap misalignment is difficult to detect when inserting all straps as an assembled interlocked unit into a fixture, and if a strap is damaged the entire grid assembly must be removed to replace the damaged strap. Furthermore, the grid cell pitch typically shrinks during welding, which can have the effect of “locking” the grid assembly into the fixture. High fixture removal forces are common with existing spacer grid welding fixture plates. Overcoming these forces may result in damage to the grid, and also generates tooling wear.